Field of Technology
This disclosure relates to a light source, and more specifically to a near-infrared light source for use in near infrared transmission and reflection spectroscopy.
Background
Optical transmission and reflection spectroscopy in the short wavelength near-infrared range of 700-1050 nm has been used extensively in the past for important applications including identification of solid, powdered, and liquid materials and compounds, and quantification of the concentration of specific chemical compounds in solids, powders, and liquids. Spectroscopic analysis in the short wavelength near-infrared range is based on molecular vibrational overtone absorptions and light scattering effects in the 700-1050 nm wavelength range. The standard light source that is used for both reflection and transmission sampling modes in most of the near-infrared spectrometer systems that operate in the 700-1050 nm range is the tungsten-halogen lamp, which emits over a very broad wavelength range of about 350-3,000 nm.
Single crystal Ti+3-Sapphire circular and rectangular cross-seciont rods and disks have been used as a broadly tunable laser medium. This material, which emits photoluminescence in the 600-1050 nm range and absorbs light from 400-620 nm, with 90% of the peak absorption within the range of 465-510 nm, has a high photoluminescent quantum efficiency in the range of 0.8 to 0.86. The excitation spectrum for Ti-Sapphire is expected to be very close to that of the absorption spectrum based on reported excitation and absorption spectra for ruby (Cr+3 doped Sapphire). Examples of other types of solid state broad-band light sources used for optical spectroscopy and microscopy include: white LEDs, and laser based nonlinear plasma solid state sources, and nonlinear photonic crystal fiber based continuum lasers. With the exception of the white LED, which does not emit at wavelengths longer than 700 nm, these other broad band solid state light sources are not always suitable due to size and expense.
U.S. Pat. No. 6,836,502 (Canady et al., Dec. 28, 2004) describes a design for a broadband near-infrared light source for spectroscopy applications that consists of a LED excitation source together with a phosphor element based on either a CdS semiconductor crystal or polycrystal, or one or more fluorescent organic dyes dissolved in a clear polymer block, or one or more sizes of fluorescent quantum dots embedded in a clear polymer block. These three phosphor element design options have some drawbacks. Although CdS has a broad luminescence emission spectrum that covers a near-infrared wavelength range, it has been reported to have low photoluminescence quantum efficiencies of 0.22, with an even lower quantum efficiency implied from reported temperature dependence of photoluminescence between room temperature and low temperatures. Phosphor elements based on organic dyes have problems with photochemical degradation and also from reabsorption of luminescence due to insufficient separation between the peak absorbance wavelengths and the emission wavelength range for the dyes that emit at the longer wavelengths in the 850-1050 nm range. The organic fluorescent dyes only emit light in limited width wavelength bands of about 100 to 150 nm, which requires a mixture of several dyes to cover the desired 350 nm range of 700-1050 nm. Disadvantages of quantum dots include a very high material price, and also the close proximity of the absorption bands to the emission bands, which leads to reabsorption of luminescent emission light. Such reabsorption results in lowering of the effective quantum efficiency. Like the organic dyes, quantum dots have limited spectral emission bands on the order of about 100-150 nm which requires a mixture of several sizes of quantum dots to cover desired the 700-1050 nm range.